EP0413231A2 - Process for the production of an inorganic sintered body - Google Patents

Abstract

Inorganic sintered bodies are produced by deformation of a mixture of a sinterable inorganic powder and polyoxymethylene as a binder to give a green compact. The binder is removed by treating the green compact in a gaseous, acid-containing atmosphere or in a gaseous, boron trifluoride-containing atmosphere. The green compact thus treated is then sintered.

Description

It is known to produce molded parts from inorganic materials by mixing a ceramic powder or a metal powder with a thermoplastic resin, shaping the mixture into a green molded body, removing the thermoplastic resin and then sintering this emaciated green body into the molded body. According to EU-PS 125 912, the emaciated green body is processed to its essentially final shape before sintering. As thermoplastic resin or binder e.g. Polystyrene, polypropylene, polyethylene and ethylene-vinyl acetate copolymers are used. These binders are removed from the green body by heating to temperatures of 300 to 550 ° C for 3 to 8 hours. The binders are thermally split. It must be heated very carefully and slowly to these temperatures so that the green body is not damaged by uncontrolled decomposition of the organic substance and the associated cracking. For this reason, the heating temperature should only be 4 ° C / hour. In US Pat. No. 4,671,912, even lower heating temperatures of 1 to 2 ° C./hour are recommended, at least until half of the binder has been removed. These long heating-up periods of several days greatly reduce the economics of these processes.

To accelerate the heating-up times, EP-PS 115 104 recommends using a mixture of an oxidized paraffin wax or an oxidized microcrystalline wax with a higher fatty acid as the binder. EP-PS 114 746 proposes a polyacetal as a binder.

A disadvantage of all of these processes, in which thermoplastics or waxes are used, is that the green body has to be heated to temperatures above the softening point of the binder for the pyrolytic removal of the binder, whereby there is a risk of deformation. To avoid such a deformation, it is therefore proposed in US Pat. No. 4,708,838 and in JP-OS 62/12674 to embed the green body in a ceramic powder with high thermal stability.

However, it is also known not to remove the binder from the green compact pyrolytically but by extraction with a solvent. According to JP-OS 62/278160, supercritical solvents are used Carbon dioxide at 60 ° C and a pressure of 200 kg / cm², according to EP-PS 206 685 liquid carbon dioxide at temperatures from -30 ° C to 31.1 ° C used. However, special equipment is required to carry out these processes.

The present invention had for its object to provide a method for producing an inorganic sintered molded part by shaping a mixture of a sinterable inorganic powder and a polyoxymethylene as a binder by injection molding or extrusion into a green body, removing the binder and sintering, which does not have the disadvantages shown above has and in particular allows rapid removal of the binder from the green compact without disadvantageous changes to the green compact.

It has now been found that this object can be achieved by removing the polyoxymethylene by treating the green body in a gaseous acidic or gaseous boron trifluoride-containing atmosphere.

In detail, the procedure according to the invention is such that a sinterable inorganic powder with the polyoxymethylene e.g. mixed in the form of granules. Inorganic powders include oxidic ceramic powders, e.g. Al₂O₃, ZrO₂, Y₂O₃, but also non-oxide ceramic powders such as SiC, Si₃N₄ or metal powders such as Si, Fe with grain sizes of 0.1 to 50 µm are used. Mixtures of these substances can of course also be used. Both homopolymers and copolymers with molecular weights of 25,000 to 75,000 are used as polyoxymethylene. Among copolymers are the polymers of trioxane with e.g. Ethylene oxide, 1,3-dioxolane or butanediol formal understood in quantities of e.g. 2 to 4 mass% can be present.

In addition to the inorganic powders, the mixtures to be shaped advantageously contain 10 to 30% by weight of polyoxymethylene as a compromise between the desired flowability of the mixture and a high packing density of the powder. In addition to the powders, the mixtures can also contain inorganic fibers or whiskers, e.g. from Al₂O₃, SiC or Si₃N₄.

In addition, the mixtures can also contain wetting agents, plasticizers or other auxiliaries which have a favorable effect on the rheological properties of the mixtures during deformation.

The masses to be shaped are produced in the usual way in a kneader or extruder at temperatures of 150 to 200 ° C. If desired, the mass can be allowed to cool and then granulated.

The usual screw and piston injection molding machines can be used for the molding by injection molding and can be shaped at temperatures from 175 to 200 ° C and pressures from 3,000 to 20,000 kPa into a mold which is kept at 20 to 80 ° C.

The demolded green compacts are then treated according to the invention in a (gaseous) acidic atmosphere for the purpose of removing the binder. (Gaseous) acidic atmospheres can be understood both as pure acids which are in gaseous form at the treatment temperatures, but also as mixtures of acids with a carrier gas. Air or nitrogen or noble gases can be considered as carrier gases. Suitable acids are the inorganic acids which are gaseous at room temperature, e.g. the hydrogen halide, hydrogen sulfide or such acids which are appreciably evaporable at the treatment temperatures, e.g. Nitric acid.

Suitable organic acids are in principle those acids which have a boiling point at normal pressure of below 130 ° C, e.g. Formic acid, acetic acid or trifluoroacetic acid.

Instead of the gaseous, acidic atmosphere, the demolded green compacts can also be treated in an atmosphere containing gaseous boron trifluoride for the purpose of removing the binder. Gaseous atmospheres containing boron trifluoride can be understood to mean both pure boron trifluoride, but it can also be understood to mean mixtures of boron trifluoride with a carrier gas. Air or nitrogen or noble gases can be considered as carrier gases.

Instead of boron trifluoride, it is of course also possible to use adducts of boron trifluoride which can be reversibly split back into the starting components at the treatment temperatures and without decomposition of the components. The addition compounds of boron trifluoride with ethers, e.g. Dimethyl ether, diethyl ether, dibutyl ether and tert-dibutyl methyl ether.

The green compact is brought into contact with the acidic or BF₃-containing atmosphere at temperatures of 20 to 150 ° C. Temperatures of 70 to 130 ° C. are preferably used. From the concentration of the acid or the BF₃ in the gas and the temperature used depends on the duration of treatment, with increasing temperature and increasing concentration of the acid shorten the duration of treatment. If a carrier gas is used, this is first passed, for example, through the heated acid or through the BF₃ or BF₃ adduct and thus loaded with the acid or the BF₃ or the BF₃ adduct. The loaded carrier gas is then brought to the treatment temperature, which should advantageously be at least 10 ° C. above the loading temperature in order to avoid condensation. After leaving the treatment room, the acid or the BF₃ from the gas mixture, which in addition to formaldehyde and trioxane also contains other decomposition products of polyoxymethylene, can be conditioned for reuse.

The green body thus freed from the binder is then sintered in the usual way at the temperatures specific to the material used.

The process according to the invention has the advantage that no crack formation can be observed due to the careful removal of the binder. Because of the low temperatures, which are below the softening point of the binder, the green body is not undesirably deformed and the green body does not have to be embedded in a complex manner. Another advantage is the very short duration of the treatment for removing the binder compared to the prior art.

example 1

25 g of a polyoxymethylene copolymer made from trioxane with 2% by weight of butanediol formal are mixed with 119 g of an Al₂O₃ powder with an average grain size of 0.7 μm and 2.4 g of polyethylene glycol as a lubricant and processed into granules in a laboratory kneader. This granulate was processed in an injection molding machine into sticks with the dimensions 6 x 4 x 50 mm.

A stick was treated at a temperature of 130 ° C for 2 hours with a gas mixture obtained by passing 20 l / h of nitrogen through 65% by weight nitric acid heated to 110 ° C.

The weight loss of the stick was 16.6% by weight, essentially corresponding to the polymer content of the green body used.

The crack-free green body was then sintered at a temperature of 1600 ° C. to form a crack-free sintered body. The density of the sintered body was 3.65 g / cm³.

Example 2

75 g of a polyoxymethylene copolymer made from trioxane with 2% by weight of butanediol formal are mixed with 270 g of a Si₃N₄ powder with an average grain size of 0.8 μm and 5 g of polyethylene glycol as a lubricant and processed into granules in a laboratory kneader. This granulate was processed in an injection molding machine into sticks with the dimensions 6 x 4 x 50 mm.

A stick was treated at a temperature of 110 ° C for 7.5 hours with a gas mixture obtained by passing 20 l / h of nitrogen through formic acid heated to 90 ° C.

The weight loss of the stick was 19.2% by weight, essentially corresponding to the polymer content of the green compact used.

Example 3

The green body was treated as described in Example 2, except that instead of formic acid, it was treated for 4 hours at 80 ° C. with pure HCl gas. The weight loss was 17.9% by weight.

Example 4

The green body was treated as described in Example 2, but the green body was treated with a gas obtained by passing 20 l / h of nitrogen through a 35% by weight hydrochloric acid at 90 ° C. The treatment lasted 2 hours, the weight loss after this time was 20.2%.

Example 5

450 parts by volume of a polyoxymethylene copolymer from trioxane with 1.5% by weight butanediol formal were mixed with 550 parts by volume of a mixture of 93% by weight Si₃N₄ powder, 2% by weight Al₂O₃ powder and 5% by weight Y₂O₃ powder with an average grain size of the powders of 0.5 to 1 µm and the addition of 40 parts by volume of polyethylene glycol with an average molecular weight of 400 g / mol as a lubricant at 180 ° C. in a laboratory kneader and then comminuted into granules. This granulate was processed by means of an injection molding machine into disks with a diameter of 80 mm and a thickness of 4 mm. The mass temperature was 190 ° C, the mold temperature 105 ° C.

The subsequent debinding was carried out in a largely sealed drying cabinet with a volume of 50 l. The atmosphere inside the drying cabinet, a fan circulates strongly in order to achieve a constant temperature in the entire volume and good heat transfer to the body to be deboned.

The drying cabinet was loaded with one of the injection molded panes so that the pan was hung on a wire that was passed up through the housing of the drying cabinet and connected to a scale to continuously measure the weight loss.

The cabinet was then flowed through for 20 minutes with a nitrogen stream of 400 l / h in order to displace the air to an O₂ content of less than 1-2% by volume. At the same time, the atmosphere in the drying cabinet was heated to 150 ° C. The debinding was started with the addition of 10 l / h of BF₃ to the nitrogen stream of 400 l / h, so that the concentration of the BF₃ in the dosage was 2.5% by volume.

With an initial weight of the disc of 46.2 g, the binder fraction is 25.3% by weight, corresponding to 11.7 g. The following table shows weight loss as a function of time: Time (min) Weight loss (g) Degree of debinding (%) 0 0 0 5 3.12 26.7 10th 5.32 45.5 15 6.81 58.2 20th 7.96 68.0 25th 9.18 78.5 30th 10.59 90.5 34 11.70 100 After the binder was removed, the disc showed no cracks or any dimensional change

Example 6

The disks produced according to Example 5 were treated as described in Example 5, but with the difference that 50 g of boron trifluoride diethyl etherate per hour were metered into the nitrogen stream (400 l / h) instead of the boron trifluoride and the mixture within a 5 m long tube wound into a spiral with an inner diameter of 3 mm stainless steel was heated to 170 ° C. The gas was fed into the drying cabinet, in which a temperature of 120 ° C. was reached.

With an initial weight of the disc of 100.0 g, the binder content is 12.7% by weight, corresponding to 12.7 g. The following table shows weight loss as a function of time. Time (min) Weight loss (g) Degree of debinding (%) 0 0 0 5 3.71 29.2 10th 5.79 45.6 15 7.01 55.2 20th 8.53 67.2 25th 10.01 78.8 30th 11.80 92.9 35 12.11 95.4 40 12.32 97.0 The disk was then kept at 400 ° C. within 1 h. The weight decreased by a further 0.38 g, corresponding to a degree of debinding of 100%. The disc showed no cracks or any dimensional change.

Claims (4)

1. A process for producing an inorganic sintered molded part by shaping a mixture of a sinterable inorganic powder and polyoxymethylene as a binder by injection molding or extrusion into a green body, removing the binder and sintering, characterized in that the polyoxymethylene is treated by treating the green body in a gaseous, removed acidic or gaseous boron trifluoride atmosphere. 2. The method according to claim 1, characterized in that one carries out the treatment at temperatures of 20 to 150 ° C. 3. Process according to Claims 1 to 2, characterized in that the acids used are nitric acid, hydrohalic acids and / or organic acids with a boiling point below 130 ° C. 4. Process according to Claims 1 to 3, characterized in that formic acid is used.